An image sensor is a semiconductor device that converts an optical image into an electrical signal. Image sensors are typically classified as charge coupled devices (CCD) or Complimentary Metal Oxide Semiconductor (CMOS) image sensors (CIS).
According to the related art, photodiodes are formed on a substrate through an ion implantation scheme. However, as photodiodes become smaller to increase the number of pixels without enlarging the chip size, an area of a light receiving section is scaled down, thereby leading to degradation of the image quality.
In addition, since the stack height is not sufficiently reduced corresponding to the area reduction of the light receiving section, the number of photons input into the light receiving section can be decreased due to the diffraction of light, often referred to as “Airy Disk”.
A back side illumination image sensor can be provided, which receives light through a wafer back side to minimize step difference at an upper portion of a light receiving section and removes light interference caused by metal routing.
FIG. 1 is a cross-sectional view showing the procedure for forming a back side illumination image sensor according to the related art.
A light receiving device and metal lines are formed on a front side of a substrate and then the substrate is subject to a back grinding process such that a predetermined portion of the back side of the substrate can be removed. That is, the back side of the substrate is ground by a predetermined thickness in order to adjust the distance between an external module and an optical lens.
However, according to the back side illumination image sensor of the related art, a silicon on insulator (SOI) wafer is used as a donor wafer having a light receiving device and a circuit section, and then the SOI wafer is bonded to a handle wafer. Then, a back side thinning process is performed with respect to the donor wafer.
In the back side thinning process for the donor wafer according to the related art, a back side grinding process is performed with respect to the donor wafer such that a silicon layer having thickness of several tens of tm may remain on a buried oxide (BOX) layer. Then, an etch back process is performed, thereby completing the back side thinning process.
However, according to the related art, the expensive SOI wafer is used as the donor wafer, leading to a high manufacturing cost.
In addition, referring to FIG. 1, wafer edge thinning may occur when the back side grinding process is performed with respect to the donor wafer. Thus, when the subsequent etch back process is performed, chip failure may occur at a wafer edge part, thereby significantly lowering the economical efficiency.
Furthermore, a wafer center part is exposed to plasma damage when the etch back process is performed with respect to the wafer having a thickness of several tens of μm, thereby degrading sensor performance.
In addition, a removing process must be performed with respect to a photodiode area of a back side of a silicon substrate. This leads to the generation of many defects on the surface of the silicon substrate through the process of removing the back side of the silicon substrate. Accordingly, a leakage characteristic is degraded, so that CIS image characteristics may be degraded.
Moreover, the photodiode is deposited by using amorphous silicon (Si). Otherwise, after a readout circuitry is formed on a silicon substrate and the photodiode is formed on another wafer, the photodiode is formed over the readout circuitry through a wafer-to-wafer bonding scheme to form an image sensor (hereinafter, referred to as a “3D image sensor”). In this case, the photodiode is connected with the readout circuitry through a metal line.
However, according to the related art method for manufacturing the 3D image sensor, the wafer-to-wafer bonding must be performed with respect to the wafer having the readout circuitry and the wafer having the photodiode. Due to the wafer-to-wafer bonding, electric connection between the readout circuitry and the photodiode may not be ensured. For instance, according to the related art, the metal line is formed on the readout circuitry and then the wafer-to-wafer bonding is performed to allow the metal line to make contact with the photodiode. At this time, the metal line may not firmly come into contact with the photodiode, and there is difficulty in formation of an ohmic contact between the metal line and the photodiode. In addition, a short may occur in the metal line electrically connected to the photodiode. Though much research and many studies have been conducted to prevent such a short from occurring, there is a need in the art for an improved method of manufacturing a back side illumination image sensor.